Method of quantifying hydrocarbon formation and retention in a mother rock

ABSTRACT

The method according to the invention allows the formation of oil and the retention phenomenon in the mother rock to be modelled. Organic matter characterization experiments are used to establish the molecular model (MM) of the initial sample (E). The thermal cracking reaction of this molecular model is reproduced by dynamic molecular simulation computations with a reactive force field (RMD) and validated by comparison with experimental data. The reaction mechanism obtained (SR) allows to carry out a kinetic study (C) by variation of the temperature parameter. The phase equilibria (PES) of the reaction medium are determined at any time from dynamic simulation. The successive phase equilibrium assessments at various progress stages of the cracking reaction allow following the physicochemical evolution (PC) of the thermal maturation of the organic sample studied. The free hydrocarbons (liquid and gaseous) that are not retained in the solid residue can be quantified throughout numerical modelling of the sample maturation; representing, in the sedimentary basins, the hydrocarbons that are not retained in the organic matrix of the mother rock (Q). This quantity can be used as an indicator or an input value for the retention threshold in basin models.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of quantifying hydrocarbonformation and retention within a macromolecular chemical system.

2. Description of the Prior Art

The following documents, mentioned in the description hereafter,illustrate the state of the art:

-   Behar F., Vandenbroucke M., Tang Y., Marquis F., Espitalié J., 1997.    Thermal Cracking of Kerogen in Open and Closed Systems:    Determination of Kinetic Parameters and Stoichiometric Coefficients    for Oil and Gas Generation. Org. Geochem., 26, 5-6, 321-339.-   Burnham, A. K. and Braun, R. L., 1989. Development of Detailed Model    of Petroleum Formation, Destruction, and Expulsion from Lacustrine    and Marine Source Rocks. Advances in Organic Geochemisrty, 16, 1-3,    27-39.-   Burnham, A. K. and Braun, R. L., 1990. Mathematical Model of Oil    Generation, Degradation, and Expulsion. Energy and Fuel, 4, 132-146.-   Faulon, J. L., Prediction Elucidation and Molecular Modeling.    Algorithms and Applications in Geochemistry, Ph. D. Thesis, Edited    by Ecole des Mines, Paris, 1991.-   Faulon, J. L., Stochastic Generator of Chemical Structure (4)    Building Polymeric Systems with Specified Properties, J. Comput.    Chem., 2001, 22, 580-590.-   Freund, H., Walters, C. C., Kelemen, S. R., Siskin, M., Curry, D.    J., Xiao, Y., Olmstead, W. N., Gorbaty, M. L., Bence, A. E., 2005.    Predicting Oil and Gas Compositional Yields via Chemical    Structure-Chemical Yield Modeling (CS-CYM). Organic Geochemistry    Challenges for the 21st Century (Vol. 1), 22 IMOG Seville, Spain,    66-67.-   Hatcher, P. G., 1988. Dipolar-Dephasing 13C Studies of Decomposed    Wood and Coalified Xylem Tissue: Evidence for Chemical Structural    Changes Associated with Defunctionalization of Lignin Structural    Units During Coalification. Energy & Fuels 2, 48-58.-   Pepper, A. S., 1991. Estimating the Petroleum Expulsion Behaviour of    Source Rocks: A Novel Quantitative Approach. In Petroleum Migration    (Edited by England W. A. and Feed A. J.), Geological Society,    Special Publication. 59, pp. 9-31.-   Pepper, A. S., Corvi, P. J., 1995. Simple Kinetic Models of    Petroleum Formation. Part III: Modelling and Open system. Marin and    Petroleum Geology, 12, 4, 417-452.-   Pepper, A. S., Dodd, T. A., 1995. Simple Models of Petroleum    Formation. Part II: Oil to GAS cracking. Mar. Petrol. Geol., 12,    321-340.-   Ritter, U., 2003. Fractionation of Petroleum During Expulsion from    Kerogen Journal of Geochemical Exploration. 78-79, 417-420.-   Ritter, U., 2003. Solubility of Petroleum Compounds in Kerogen:    Implications for Petroleum Expulsion. Organic Geochemistry. 34,    319-326.-   Tissot, B. 1969. Revue Inst. Fr. Pétrole, 24(4), 470-501.-   Ungerer, P., 1989. State of the Art of Research in Kinetic Modelling    of Oil Formation and Expulsion. In Advances in Organic Goechemistry,    Organic Geochemistry. 16, 1-3, 1-25.-   Van Duin A. C. T, Siddharth D., Lorant F., Goddard III W. A. 2001.    ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A,    105, 9396-9409.

The insoluble organic material of the mother rock, also referred to askerogen, is a mixture of bio-organic macromolecules (notablybiogeopolymers) having aliphatic and aromatic chemical structures thatevolve in the course of geologic times with the temperature and thepressure. Thermal maturation of the kerogen in the mother rock occursthrough the agency of two main phenomena:

the first one is thermal cracking of the organic matter at the origin ofhydrocarbons; it takes place naturally in sedimentary basins, generallyat a temperature ranging between 80° C. and 200° C., and a pressureranging from 200 to 1000 bar,

the second one is the physicochemical evolution of the petroleumproducts within the mother rock that explains the retention andexpulsion of hydrocarbons from the mother rock.

These two phenomena develop within the same context and they arejuxtaposed. In basin modelling, it is important to be able tosimultaneously calibrate the amounts of hydrocarbons formed and theamounts of “free” hydrocarbons that can be expelled from the mother rockand migrate to the reservoir. The process of hydrocarbon retention inkerogen is a mechanism that controls the free hydrocarbons/expelledhydrocarbons ratio. If the retention of hydrocarbons is considered topredominantly occurs in the organic matter of the mother rock, thisretention depends on the composition of the fluids generated, on theretention capacities of the kerogen and on the volume ratio of the solidorganic matter to the liquid hydrocarbons. It is thus directly linkedwith the physicochemical nature of the kerogen and with the conversionratio (Pepper, 1991).

Kerogen cracking and retention of the products from this reaction beinginterdependent phenomena that evolve with the thermal maturation ofkerogen, it appears necessary to develop a fine analysis of the crackingreaction coupled with the retention of the products formed in thekerogen in order to be able to estimate the hydrocarbons retained in thekerogen and the hydrocarbons available for storage in reservoirs.

There are known hydrocarbon quantification models that account foreither the thermal cracking reaction, or the hydrocarbon retentionphenomenon, but rarely of both phenomena simultaneously:

empirical and mechanistic cracking models are used to quantify thehydrocarbons formed in the mother rock,

models of hydrocarbon retention in the mother rock try to explain thesegregation of the hydrocarbons when they are expelled from the motherrock.

Thermal Cracking Models

Two cracking reaction modelling methods are provided in the literature:empirical models and mechanistic models.

Empirical models are based on experiments to establish the globalstoichiometric equations that account for the mass balances observed.

These stoichiometric equations are coupled with hydrocarbon formationvelocity laws, they correspond to a series of simultaneous, independentand competitive reactions and they were developed assuming that theglobal evolution of the petroleum potential of a kerogen undermaturation is an irreversible kinetic process (Pepper and Dodd, 1995).The kinetic (E: activation energy, A: frequency factor) andstoichiometric (Xi: relative contribution of reaction i) parameters haveto be calibrated individually because the mother rocks do not generatehydrocarbons at the same rate. Artificial maturation experiments aretherefore carried out in the laboratory under controlled thermalconditions. These experiments are performed on kerogens or mother rocksand they can be of different natures. By numerical inversion of thelaboratory data, it is possible to calculate the kinetic andstoichiometric parameters. These parameters, obtained at hightemperature (300° C.-600° C.) over short times (some minutes to somedays) are then assumed to be extrapolatable for lower temperatures thanthose of the experimental conditions such as those imposed by thegeothermal gradients.

This method is currently the only means allowing providing informationon the formation of hydrocarbons compatible with basin models. However,this procedure is based on many approximations. In fact, Behar et al.'swork of 1997 showed that the differences in the experimental conditionsof the pyrolyses carried out (open or closed medium, in the presence orabsence of water or of mineral matrix, or according to the grain size ofthe sample) leads to a kinetic parameters lag and therefore to anuncertainty in the estimation of the oil window. Similarly,extrapolation of the kinetic parameters at low temperature involves thenature of the cracking mechanism does not significantly develop as afunction of temperature.

Mechanistic models are not based on stoichiometric equations but onelementary (radical) reactions to simulate the thermal degradation ofcomplex macromolecules and to reproduce the distribution of thehydrocarbons formed (Freund et al., 2005). Elaboration of these modelsstarts by modelling the initial macromolecule. This modelling isconstrained by experimental data relative to the structural propertiesof the sample. It is based on the distribution of the functional groupsin the molecule for establishing the probable structure thereof. Oncethe initial macromolecule is defined, elementary reactions are appliedto the structure so as to simulate the formation of the thermaldegradation products. Each elementary reaction has its own kineticproperties, valid whatever the temperature scale. It is thus possible tosimulate thermal maturation both under laboratory conditions and undergeologic conditions.

The advantage of this approach is that it minimizes the uncertainty onthe reaction velocities extrapolated to the geologic conditions. On theother hand, the complexity of these models makes elaboration of thereaction mechanism difficult. Finally, and above all, the very largenumber of reactions in these models is incompatible with current basinsimulators.

Retention Models

Physical models have been provided in order to estimate the proportionof expelled hydrocarbons as a function of the kerogen conversion rate.Ungerer's expulsion model (1989) sets a threshold corresponding to aconversion rate for which the hydrocarbons formed in the mother rock areexpelled. In this connection, Pepper (1991) considers correlating “thepetroleum expulsion efficiency” (PPE) with the initial petroleumpotential of the mother rocks considered. These models reproduce more orless accurately (according to the mother rock type) the quality of thehydrocarbons of the reservoir. They consider the mother rock in twostates only, before and after expulsion of the hydrocarbons, withouttaking into account the qualitative evolution of the mother rock or theexpulsion kinematics.

Later, Ritter (2003) provided a retention model based on the solubilityof the hydrocarbons in kerogen. He established an empirical relationbetween the swelling ratio and the Hildebrand solubility parameter, foreach type of organic matter. This relation defines a retention ratio foreach group of compounds. Finally, this model confirms the fractionationsequence observed in nature, except for branched aliphatic hydrocarbons.The polymer solubility theory and this model thus do not totally explainthe hydrocarbon composition differences between mother rock extracts andreservoir oils, observed in petroleum systems. Similarly, this modeldoes not explain the great accumulation of aliphatic hydrocarbons incoals. This model has two limitations: the first one is that the valuesof the swelling ratio are filed according to their chemical class butthey are not normalized. Thus, the model does not respect the massconservation principle. This generates too high retention thresholds andthe sum of the compositions is above 100%. The second drawback is due tothe fact that the swelling phenomenon involves swelling of the organicmatrix. Now, there is little chance that this swelling occurs in rockssubjected to high overpressures.

In conclusion, the expulsion and retention models provided to dateinvolve possible mechanisms and they are developed with more or lessassumptions, which leads to more or less realistic approaches. In fact,the structure of kerogen and the nature of the effluents vary with themother rock maturity, therefore thermal cracking and expulsion areindissociable processes.

The method according to the invention allows quantification of theformation and the retention of hydrocarbons in a mother rock from a newtype of simulation. This simulation type is based on a dynamic molecularmodelling technique coupled with a reactive force field. As in the caseof mechanistic models, this approach requires as the starting point a“molecular” representation of the structure of the kerogen. The methodaccording to the invention does not require writing hundreds of a priorireactions: the reaction mechanism is not an input datum, it becomes aresult of the dynamic simulation. As in the case of radical mechanisms,this new technique is applicable in any thermal regime.

SUMMARY OF THE INVENTION

The invention relates to a method of quantifying the formation and theretention of hydrocarbons within a macromolecular chemical system,comprising construction of a molecular model of the system by means ofexperimental characterizations of a sample of the system, andcomprising:

defining a thermal maturation reaction mechanism for a macromolecularchemical system by subjecting the molecular model to a molecular dynamicsimulation associated with a reactive force field;

quantifying the free hydrocarbons formed throughout the thermalmaturation of the macromolecular system by:

a) determining a physicochemical evolution of the molecular mixturedefined by the reaction mechanism, by calculating phase equilibria,directly from the molecular dynamic simulation; and

b) determining the thermal decomposition kinetics of the macromolecularchemical system by kinetic study from said reaction mechanism.

The present invention comprises a method of modelling the thermalreactivity of macromolecular chemical systems whose molecular structureis unknown or not well known.

In particular, it can be used within the context of basin modellingwhere the method allows parameterization of the proportion ofhydrocarbons formed and possibly retained in the organic matrix of themother rock.

In the description hereafter, what is referred to as “macromolecularchemical system” or “macromolecular system” is a system consisting of atleast one organic macromolecule, for example the organic matter of themother rock.

The “reaction mechanism” is understood to be all of the chemicalreactions describing the thermal maturation of each macromolecule. Areaction mechanism is associated with a set of quantitative andqualitative data that characterize the reagents, the products and thevelocities of the chemical reactions.

A “force field” is a set of parametrized equations that describe thevarious contributions to the total potential energy of a chemical system(notably Van der Waals, Coulomb, torsional energies).

In the method according to the invention, the molecular model and thereaction mechanism are advantageously validated by comparison of theresults from the reactive molecular dynamic simulation with experimentalmeasurements of the thermal maturation of the macromolecular chemicalsystem.

Determination of the phase equilibria is preferably performed bycalculating at any time the total energy of each molecule subjected tothe force field, then by successive assessments in order to quantify thehydrocarbon retention in the macromolecular chemical system.

The kinetic study is preferably carried out by varying the temperatureof the reaction mechanism.

In an embodiment, the macromolecular chemical system represents theorganic matter of a mother rock of a petroleum reservoir.

The invention also relates to a method of simulating the genesis of asedimentary basin, wherein the formation and the retention ofhydrocarbons within a mother rock containing kerogen are modelled, andwherein the following stages are carried out:

defining by means of experimental characterizations a molecular model ofthe kerogen of the mother rock;

defining a reaction mechanism by subjecting the molecular model to amolecular dynamic simulation associated with a reactive force field;

quantifying the evolution of the kerogen to heavy products andhydrocarbons, by carrying out a kinetic study of the thermaldecomposition of the kerogen from said reaction mechanism;

quantifying the hydrocarbon retention within the mother rock by carryingout assessments on the fractions linked to the organic matrix of themother rock and on the free fractions from the phase equilibriadetermined by molecular dynamic simulation; and

using as the input data in a basin simulator the amounts of hydrocarbonsformed and the amounts of hydrocarbons that are not retained in themother rock, that can be expelled from the mother rock and migrate to apetroleum reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the method according to the inventionwill be clear from reading the description hereafter of a non limitativeembodiment example, with reference to the accompanying figures wherein:

FIG. 1 shows the principle of the method according to the various spaceand time scales of the phenomena modelled;

FIG. 2 illustrates the sequence of the stages of the method for reachingevaluation of the hydrocarbons formed and retained in the mother rock;

FIG. 3 illustrates the lignite molecular characterization protocol;

FIG. 4 illustrates the result of the reactive dynamic simulationsaccording to the invention on three molecular models of lignitefragments;

FIG. 5 illustrates the lignite thermolysis mechanisms; and

FIG. 6 is a cross-sectional view of an oil development field.

DETAILED DESCRIPTION

The method according to the invention numerically models the thermalmaturation of the organic matter of the mother rock or of any othermacromolecular system, and the retention of the products from thisreaction in the residual organic matrix, and to possibly extrapolate theresults to the basin scale.

FIG. 1 shows the principle of the method as a function of the variousspace scales (D(m): Distance in meter) and time scales (t(s): time insecond). The initial kerogen (or any other macromolecular structure)sample (E) is experimentally characterized, which allows determinationof a molecular model of the structure (MM (Å, 10⁻¹⁵ s)). This molecularmodel is the input datum of the dynamic simulations coupled with areactive force field (RMD (Å, 10⁻¹⁵ s)). These simulations model thethermal cracking reaction of the molecular model under the givenconditions, and they are validated by comparison with the experimentalthermal maturation data. At several stages of the simulated reaction,phase equilibrium calculations (PES (μm, 10⁻¹⁵ s)) are carried out inorder to establish a physicochemical assessment of the reaction medium.The free hydrocarbons and those retained in the solid residue areestimated on the molecular scale from all of these calculations, thenextrapolated to the geologic conditions (Q (Km, Ma)).

The method can be broken down into three major stages:

-   -   1—-Determination of the nature and of the molecular structure of        the macromolecular chemical system (kerogen for example)    -   2-Reactive dynamic molecular simulation    -   3-Thermodynamic study and kinetic study

These stages are diagrammatically shown in FIG. 2; from laboratory dataon sample (E), a molecular model (MM) of the sample is established andintroduced in a reactive dynamic simulation (RMD). The results of thereactive dynamic simulation are validated (V) by comparison with theexperimental data. The reactive dynamic simulation (RMD) allowsconstruction of a reaction mechanism (SR) and description of thephysicochemical evolution of the retention (PC) from the phaseequilibria determination. From the reaction mechanism, the thermaldecomposition reaction kinetics is determined, notably by thermalcracking (C). Coupling the kinetic study and the physicochemical studyallows quantification of the hydrocarbons produced and the “free”hydrocarbons (Q).

First Stage: Determination of the Molecular Structure and of the Natureof the Organic Matter—Elaboration of a Molecular Model

The first stage of this method determines the structure and the natureof the macromolecular system (notably organic matter of the mother rock)that will be used as initial data for modelling the thermal maturationreaction. For example, many organic matter characterization techniques,such as elementary analysis (AE), Fourier transform infraredspectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR) andX-ray diffraction (DRX), can be used to quantify the various functionsand to know the spatial distribution of the molecular structure.Similarly, indirect characterization techniques such as the study ofpyrolysis products are also an efficient means of determiningmacromolecular structures. Using molecular models such as Signature(random macromolecular generator, Faulon, 1991 and 2001) associated withthe experimental characterization data can allow obtaining a morerealistic macromolecular system. The first stage of the method accordingto the invention allows to establish a molecular model of themacromolecular system.

Second Stage: Reactive Dynamic Molecular Simulation

The cracking reaction of the molecular model of the macromolecularsystem is reproduced by dynamic molecular simulation calculationscoupled with a reactive force field (RMD). These simulations arecompared with experimental thermal maturation data (quantitative data(conversion, mass balance, elementary analysis . . . ) and qualitativedata (product characterization (by gas chromatography coupled with amass spectrometer GC-MS, magnetic resonance NMR, Fourier transforminfrared spectroscopy FT-IR, X-ray diffraction DRX . . . ) frompyrolyses in open or closed media, or other thermal maturationexperiments depending on the simulation conditions). The similaritybetween the experimental and numerical results validates both themolecular model of the initial sample and the reaction mechanismprovided by the simulation.

Dynamic simulations are three-dimensional molecular modellings allowingrepresentation of a set of molecules in a force field describing theenergy status of the medium. In a given volume, a system of moleculeshaving n atoms and m bonds will turn into a system keeping a number n ofatoms but evolving towards a number m′ (different from or equal to m) ofbonds.

Van Duin et al. (2001) provide a reactive force field called “ReactionForce Field” (ReaxFF). This force field is an intermediate approachbetween the quantum models and the non-reactive force fields.

The two particular features of this field are that its energy functionis based on the order of the bonds between atoms, and that there is acontinuity between the binding energies and the non-binding energies.The result is a rather complex energy function that incorporates thepartial energy contributions allowing to describe the various types ofbonds, as well as the cleavages and the formation of bonds:E _(total) =E _(bond) +E _(over) +E _(under) +E _(val) +E _(pen) +E_(tors) +E _(colj) +E _(vdw) +E _(coulomb)with:

-   -   E_(bond)=binding energy    -   E_(under)=undercoordination energies    -   E_(over)=overcoordination energies    -   E_(val)=valence energy    -   E_(pen)=penalty term (associated with E_(over), E_(under) in the        case of allenes)    -   E_(tor)=torsional energy    -   E_(conj)=conjugation energy    -   E_(vdw)=van der Waals energy    -   E_(coulomb)=Coulomb energy

A parametrized analytical function relating the energy to the order ofthe bonds involved corresponds to each energy term. For example, thebinding energy between two atoms i and j is written as follows:E _(bond) =−D _(e) ·BO _(ij)·exp(p _(be,1)(1−BO _(ij) ^(p) ^(be,1) ))where BO_(ij) is the order of the bond between i and j, D_(e) andp_(be,1) are parameters calibrated for different atom pairs. Accordingto Pauling's principle, the bond order itself depends on the relativedistance between the atoms. In the case of pluriatomic systems,calculation of BO_(ij) between two atoms takes account of the atomicenvironment. Van Duin et al. (2001) developed a calculation mechanismrelating the bond orders to the relative positions of the atoms in spacewhile taking account of all of these interactions.

To date, ReaxFF allows carrying out dynamic simulations on systemsconsisting of at least one of the following atoms: C, H, O, N, S, Si,Pt, Zr, Ni, Au, V, Bi, Ti, Mo. The parameters associated with thedifferent energy terms are calibrated by numerical inversion of both theexperimental data (generally reaction enthalpy) and the theoretical datacalculated by means of quantum methods.

Concerning the dynamic simulation as such, ReaxFF comes in form of asoftware including both the force field (in form of an auxiliary file)and the molecular dynamics engine. This engine takes up the basicprinciples of dynamic simulation wherein the motion of the atoms isdescribed by the Newtonian mechanics in the three-dimensional space:$\left\{ \begin{matrix}{{\overset{->}{F}}_{i} = {{m_{i}\frac{\mathbb{d}^{2}{{\overset{->}{r}}_{i}(t)}}{\mathbb{d}t^{2}}} = {- \frac{\mathbb{d}\overset{->}{E}}{\mathbb{d}{r_{i}(t)}}}}} \\{\overset{->}{E} = {{f\left( {BO}_{ij} \right)} = {f^{\prime}\left( {r_{1},\ldots\quad,r_{n}} \right)}}}\end{matrix}\quad \right.$with: F_(i)=force exerted on atom i

-   -   r_(i)=trajectory of atom i.

Thus, dynamic simulations with force field ReaxFF allow reproduction ofthe intra and intermolecular mechanisms of chemical reactions.

FIG. 4 illustrates the main parameters of the reactive dynamicsimulations on three lignite fragments. First (SI), one or moremolecules (N₀ atoms and M₀ atomic bonds) are constructed in threedimensions in a box of predetermined dimensions. It is possible to varythe volume parameters of the box, the pressure and/or temperatureparameters during integration of the model as a function of time. Asuccession of states of the system (SF) evolving as a function of timein the force field (N₀ atoms but Mt (≠M₀) bonds) is obtained.

Third and Fourth Stages: Thermodynamic Study and Kinetic Study

The dynamic simulation results allow establishing a reaction mechanismand they can be interpreted or developed according to two approaches:

physicochemical approach of the reaction by studying the phaseequilibria: thermodynamic study,

kinetic approach of the decomposition reaction for quantifying theformation of the products predominantly formed or the dissociation ofthe initial model kerogen: kinetic study.

Physicochemical study (PC): at each time interval, the dynamicsimulation calculates the total energy of each molecule of the forcefield. From these energies, it is possible to sort out the moleculesaccording to their physical state (liquid, solid or gaseous). Thephysical evolution of each molecule can then be monitored duringsimulation of the reaction. The successive phase equilibrium assessments(PES) allows following the physicochemical evolution of the thermalmaturation of the sample studied. This thermodynamic analysis allowsstudy and qualification of the thermolysis products that are independentof the “residue” (one or more molecules in solid phase and of highmolecular mass) of the dependent products.

Kinetic study (C): the kinetic study of the reaction requires dynamiccalculations at different temperatures. At each temperature,quantitative assessments of the appearance or disappearance ofpredominant molecules are established in order to calculate the reactionvelocity (k: rate of appearance or of disappearance of a chemicalspecies). From the values of different reaction velocities of eachchemical species at the different temperatures, it is then possible tocalculate the kinetic parameters (activation energy Ea andpre-exponential factor A) and to deduce therefrom the thermaldecomposition kinetics of each molecular species in the temperaturerange studied.

Finally, from a double study (physicochemical and kinetic) of themechanisms provided by the dynamic simulations, the free hydrocarbons(liquid and gaseous), not retained in the solid residue, can bequantified throughout the numerical maturation of the sample. Theyrepresent, in sedimentary basins, the hydrocarbons likely to be expelledfrom the mother rock (Q) and they can thus be an input parameter forbasin simulators.

Example: Study of Lignitic Coal

The Australian brown coal of this example essentially consists oflignite molecules (L). Some characteristics of this coal and of relatedchemical structures are given in Hatcher's work, 1988.

Definition of the Initial Chemical System: Molecular Model

First, from experimental organic matter characterization measurementscarried out on a sample of this coal (NMR analysis, infraredspectroscopy IR, atomic composition, pyrolyses in closed circuitsPYR—FIG. 3), the chemical functions are identified and counted, then, bycomparison with the lignite structures provided by Hatcher (Hatcher,1988), one deduces that the coal can be correctly represented as anagglomerate of a form of lignite that is modelled by a macromolecule ofabout 250 atoms. In this example, the kerogen model is manuallyconstructed. For more complex kerogens, simulators such as Signature(Faulon 1999, 2001) can be used to randomly organize known units of thestructure.

Thermal Reactivity Modelling: Reactive Dynamic Simulation (RMD)

Once the initial chemical system defined, the thermal reactivity of allor part of the chemical structure deduced in the previous stage is thenmodelled. A molecular dynamic simulation algorithm associated with aparticular force field, called ReaxFF (van Duin et al., 2001), andallowing prediction of, among other things, the thermal cracking oforganic matter under given conditions, is therefore used. Examples ofsuch simulations are shown in FIG. 4. In these examples, we havemodelled the thermal cracking of three typical fragments of the lignitestructure shown in FIG. 3. For each one of the three lignite fragments(L) selected:

Number of units: 15

Volume: 3723.2 Å³

Density: 1.4 kg.L⁻¹

Number of units: 17

Volume: 5661.2 Å³

Density<1.4 kg.L⁻¹

Number of units: 21

Volume: 4870.9 Å³

Density<1.4 kg.L⁻¹

For each fragment, these simulations are carried out as follows:

a volume containing n units of the same fragment is constructed with thealgorithm,

after an initialization stage (atom velocities balancing and calculationof the system density at experimental P and T), the reactive dynamicsimulations are carried out under different thermal conditions and atconstant volume. In FIG. 4, temperature T is above 2000 K. Although thephysical duration modelled is finally of the order of some picoseconds,the calculation times can be very long (some hours to some days CPU).

At the end of a computation, in the final systems (SF), the volumecontains new molecules from the various thermal decomposition mechanismsthat have affected the initial lignite fragments (initial systems SI).The software allows identification of and counting of all of themolecules present in the volume at any time. Fine analysis of these datathus allows both prediction of the composition of the cracking productsof the initial fragments and determination of the mechanisms at theorigin thereof.

Assessment of the Reactive Dynamic Simulation Results: Achieving aReaction Mechanism

In a third stage, the results of the reactive dynamic simulations areassessed in order to extract therefrom a general coal thermaldecomposition mechanism. In the example given, this work shows (FIG. 5)that the major part of the experimentally observed pyrolysis products isobtained by means of two main cracking mechanisms affecting the lignite.These two reactions explain both the formation of water and of methane,through a defunctionalization process (mechanism 1—M1), and thedepolymerization in stages (mechanism 2—M2) of the lignite, at theorigin of the constituents of the C₁₄₊ fraction illustrated in FIG. 3.

In conclusion, a general pyrolysis mechanism for the coal studied shouldhave the form as follows:

the lignite undergoes depolymerization in fragments,

the fragments undergo defunctionalizations and give molecules such asmolecules of water (H2O), methane (CH4) and other hydrocarbons.

Kinetic Study

In order to quantify the thermolysis of lignite, a kinetic study iscarried out from the reaction mechanism predicted by reactive dynamicmodelling.

In general terms (Tissot 1969, Braun and Burnham 1989 and 1990, Pepperand Corvi 1995), the primary cracking models correspond to a series ofsimultaneous reactions, independent and competitive, describing the masstransfer between the source kerogen and the hydrocarbons. A reactionmechanism example is given below. A velocity law of order 1 that dependsboth on time and on temperature corresponds to each reaction of themechanism. Thus, the amount m of hydrocarbons formed in the course oftime is expressed by: $\begin{matrix}\left\{ \begin{matrix}{m = {X_{\inf}{\sum\limits_{i = 1}^{N}\quad{X_{i}q_{i}}}}} \\{\frac{\mathbb{d}q_{i}}{\mathbb{d}t} = {k_{i}\left( {1 - q_{i}} \right)}^{n}}\end{matrix} \right. & (1)\end{matrix}$with

-   -   q_(i)=conversion ratio of reaction i    -   X_(i)=relative contribution of reaction i    -   X_(inf)=petroleum potential    -   k_(i)=velocity constant of reaction i    -   n=order of the reaction    -   X_(i) and k_(i) obey the following laws: $\begin{matrix}        {{\sum\limits_{i = 1}^{N}\quad X_{i}} = 1} & (2) \\        {k_{i} = {A_{i}{\exp\left( {- \frac{E_{i}}{RT}} \right)}}} & (3)        \end{matrix}$        with A_(i)=frequency factor and E_(i)=activation energy for        reaction i.

Thermodynamic Study of the Molecules

From the reaction mechanism of the dynamic simulations, it is alsopossible to sort out the molecules formed according to their molecularweight and to determine their thermodynamic state. Simulators using theMonte Carlo method allow theoretical study of the phase equilibria in amolecular mixture. Using this method or an equivalent method on themolecular mixture (products and reagents of the lignite thermolysis)allows monitoring of the thermodynamic evolution of the reaction withtime. This thermodynamic analysis allows studying and quantification ofthe thermolysis products that are independent of the “residue” (one ormore molecules in solid phase and of high molecular mass) of thedependent products.

The potentialities of molecular modelling allow producing complexmolecular models (case of kerogen), to predict the thermodynamicproperties at equilibrium of these chemical structures, and to calculateparameters involved in the retention, such as viscosity, diffusion,density, phase state, electrostatic charges distribution, binding andrepelling forces, to predict the thermal reactivity of these structuresaccording to pressure and temperature, and to determine the chemicalnature of the products formed.

The method according to the invention uses a reactive dynamicsimulation, which allows prediction of the evolution of the kerogen toheavy products and hydrocarbons, as well as to perform assessments onthe fractions linked to the organic matrix and on the free fractions,thus to quantify the retention.

The method intended for modelling the thermal reactivity of complexchemical systems was applied for quantifying the formation and theretention of hydrocarbons within a complex carbon-containing system suchas the organic matter contained in a mother rock. FIG. 6 shows anexample of an oil exploration system. Zone G represents the gas window(generally temperature above 150° C. and pressure above 53 MPa). Zonerepresents the oil window (temperature generally ranging between 80° C.and 150° C., pressure ranging between 24 MPa and 53 MPa). Zone IT is thethermal immaturity zone (temperature below 80° C. and pressure below 24MPa). The “free” hydrocarbons migrate from the mother rock (RM) to thereservoir zone (RR). In the absence of cap rock (RC) at the surface, thepetroleum (P) can seep at the surface. Boreholes are drilled through therock layers to take the crude from the reservoir rock (RR).

This modelling can also find applications in other particular spheres inorder to understand and/or quantify a multiscale process that can besimulated through experiment and explained by complex and/or unknownchemical mechanisms. Non limitative examples thereof are the thermalbehavior of heavy crudes during steam or air flooding in a reservoir,and hydrocracking of asphaltenes during refining.

1) A method of quantifying formation and retention of hydrocarbonswithin a macromolecular chemical system, comprising construction of amolecular model of the system by means of experimental characterizationsof a sample of the system, comprising: defining a thermal maturationreaction mechanism for the macromolecular chemical system by subjectingthe molecular model to a molecular dynamic simulation associated with areactive force field; and quantifying free hydrocarbons formedthroughout thermal maturation of the macromolecular system by: a)determining a physicochemical evolution of a molecular mixture definedby the reaction mechanism, by calculating phase equilibria, directlyfrom the molecular dynamic simulation; and b) determining thermaldecomposition kinetics of the macromolecular system by kinetic studyfrom the reaction mechanism. 2) A method as claimed in claim 1, whereinthe molecular model and the reaction mechanism are validated bycomparison of results from the reactive molecular dynamic simulationwith experimental measurements of the thermal maturation of themacromolecular chemical system. 3) A method as claimed claim 1, whereindetermination of the phase equilibria is performed by calculating at anytime total energy of each molecule subjected to a force field, then bysuccessive assessments in order to quantify the hydrocarbon retention inthe macromolecular chemical system. 4) A method as claimed in claim 1,wherein the kinetic study is carried out by varying temperature of thereaction mechanism. 5) A method as claimed in claim 1, wherein themacromolecular chemical system represents organic matter of a motherrock of a petroleum reservoir. 6) A method of simulating genesis of asedimentary basin, wherein formation and retention of hydrocarbonswithin a mother rock containing kerogen are modelled, comprising:defining by means of experimental characterizations a molecular model ofthe kerogen of the mother rock; defining a reaction mechanism bysubjecting the molecular model to a molecular dynamic simulationassociated with a reactive force field; quantifying evolution of thekerogen to heavy products and hydrocarbons, by carrying out a kineticstudy of the thermal decomposition of the kerogen from the reactionmechanism; quantifying hydrocarbon retention within the mother rock bycarrying out assessments on fractions linked to than organic matrix ofthe mother rock and on free fractions from phase equilibria determinedby molecular dynamic simulation; and using as the input data in a basinsimulator amounts of hydrocarbons formed and amounts of hydrocarbonsthat are not retained in the mother rock, that can be expelled from themother rock and migrate to a petroleum reservoir. 7) A method as claimedclaim 2, wherein determination of the phase equilibria is performed bycalculating at any time total energy of each molecule subjected to aforce field, then by successive assessments in order to quantifyhydrocarbon retention in the macromolecular chemical system. 8) A methodas claimed claim 2, wherein the kinetic study is carried out by varyingtemperature of the reaction mechanism. 9) A method as claimed claim 3,wherein the kinetic study is carried out by varying temperature of thereaction mechanism. 10) A method as claimed claim 7, wherein the kineticstudy is carried out by varying temperature of the reaction mechanism.11) A method as claimed in claim 2, wherein the macromolecular chemicalsystem represents organic matter of a mother rock of a petroleumreservoir. 12) A method as claimed in claim 3, wherein themacromolecular chemical system represents organic matter of a motherrock of a petroleum reservoir. 13) A method as claimed in claim 4,wherein the macromolecular chemical system represents organic matter ofa mother rock of a petroleum reservoir. 14) A method as claimed in claim7, wherein the macromolecular chemical system represents the organicmatter of a mother rock of a petroleum reservoir. 15) A method asclaimed in claim 8, wherein the macromolecular chemical systemrepresents organic matter of a mother rock of a petroleum reservoir. 16)A method as claimed in claim 9, wherein the macromolecular chemicalsystem represents organic matter of a mother rock of a petroleumreservoir. 17) A method as claimed in claim 10, wherein themacromolecular chemical system represents organic matter of a motherrock of a petroleum reservoir.